Simultaneous radar observations of meter‐scale F region irregularities at and off the magnetic equator over India

Abstract

[1] Simultaneous observations of equatorial spread F irregularities made with an 18 MHz radar from Trivandrum, located at the geomagnetic equator, and a 53 MHz radar from Gadanki, located at a magnetic latitude of 6.5°N, corresponding to nearly the same longitude zone, are presented. The observations correspond to 8.3 and 2.8 m irregularities, respectively. The spread F irregularities at both the locations are found to occur nearly at the same time but are observed for longer duration at Gadanki than at Trivandrum. The spread F structures as observed in the intensity maps corresponding to Gadanki are characterized by multiple periodic plumes in contrast to a limited number of plumes observed over Trivandrum. The Doppler velocities associated with these irregularities corresponding to Trivandrum are in the range of À100-150 m s À1, whereas they are in the range of À100-250 m s À1 at Gadanki. Further, the fluctuating velocity fields are much stronger in the Gadanki observations than in the Trivandrum observations. Remarkably, the spectral widths are <100 m s À1 in Trivandrum observations in contrast to those observed at Gadanki with values as high as 300 m s À1 . The observations are compared with those made elsewhere and are discussed in the light of current understanding of the meter-scale irregularities responsible for the radar backscatter.

Intriguing aspects of F-region plasma irregularities revealed by the Gadanki radar observations during the SAFAR campaign

“…The time gaps between two consecutive plumes are between 15 min and ∼1 h. Another important aspect is the periodic descent (during 20:00-22:00 LT and 23:00-02:00 LT) and ascent (during 22:00-23:00 LT and 02:00-06:00 LT) of the base of the echoing region. Note that a large number of plumes are observed during the descent of the layer consistent with earlier observations made from Gadanki (Patra et al, 2005). Note that the highest altitude plume event occurred after the F-region sunrise (05:10 LT) and during the descending phase of the bottomside echoing envelope.…”

“…Interestingly, radar observations have also provided enormous amount of evidence that suggested the role of gravity waves as the seed perturbation for the RT instability to grow into plasma bubble (e.g., Roettger, 1973;Kelley et al, 1981;Tsunoda and White, 1981;Hysell et al, 1990;Rao et al, 1997;Patra et al, 2005). Computer simulations of gravity wave seeded RT instability have also successfully reproduced many of the radar observations (e.g., Huang and Kelley, 1996).…”

“…Usually, the irregularities associated with the bubbles that occur after the sunset decay by midnight (e.g., Patra et al, 2004Patra et al, , 2005. The fact that the plume activities continued well beyond the midnight suggests the availability of free energy for the sustenance of the F-region irregularities.…”

“…The velocities in the early phase of the event were generally larger than those observed later. For the post-sunset F-region irregularities observed at Gadanki, Patra et al (2005) reported that the upward/northward Doppler velocities are often 100 m s −1 and at times could be as high as 300 m s −1 . Thus in the present observations, the Doppler velocities were rather small.…”

Study of Atmospheric Forcing and Responses (SAFAR) campaign: overview

“…There have been several earlier studies carried out using a variety of powerful techniques such as the MST (Mesosphere-Stratosphere-Troposphere) radar, lidar operational at National Atmospheric Research Laboratory (NARL), Gadanki, which advanced our knowledge by providing valuable information on various cross related issues such as convective activities (e.g., , atmospheric waves (e.g., Ratnam et al, 2008a), temperature structure (e.g., Kumar et al, 2009), ionospheric irregularities (e.g., Patra et al, 2004Patra et al, , 2005 etc. However, the main limitations were: most of them were case studies but for a few studies (e.g., Ratnam et al, 2008b;Sridharan et al, 2009) which dealt with long-term data base, but done independently for different height regions of the atmosphere.…”